Combustion effluents may be a source of air pollution when discharged into the is atmosphere. Accordingly, the effluents or waste products are treated to remove certain contaminates. One contaminate is nitrogen dioxide (NO2). Certain chemical manufacturing processes, such as nitric acid synthesis, produce waste effluents which contain NO2. Another source of NO2 is from NO formed by combustion processes, which is produced by internal combustion engines in automobiles, stationary sources, such as power plants, process furnaces, incinerators and the like. In these combustion processes, part of the oxygen combines with atmospheric nitrogen in the flame rather than with the fuel. Because NO is the only oxide of nitrogen that is stable at high temperatures, NO is the predominant oxide of nitrogen formed by combustion. Under ambient conditions, the equilibrium 2NO+O22 NO2 favors NO2. However, at the concentrations of nitrogen oxides normally found in combustion waste gases, this reaction is typically a slow process. Hence, NO formed by combustion is generally discharged into the atmosphere as NO and only subsequently is converted to NO2.
There have been considerable efforts to determine effective ways to remove oxides of nitrogen (NOx) from combustion waste gases. Ammonia is commonly used as a reactant for the removal of nitrogen oxides from gas streams. When it is injected, it reacts with nitric oxide (NO) to form N2 and H2O and thereby reduces the emissions of the undesirable nitrogen oxides. It is typically used in concentrations similar to the NO concentration.
Two common methods are used to speed the reactions between ammonia NH3 and NO. In one method, high temperatures of about 871° C. to 1038° C. are used to speed the reaction. After this reaction, the gases, if they are from combustion in a boiler, pass through several heat exchange devices and eventually exit the stack at temperatures in the range of about 132° C. to 188° C. The gases from some older boilers may exit the stack at higher temperatures, but for efficiency it is necessary to have low stack temperatures. This type of process is known as Thermal DeNOx or Selective Non-Catalytic Reduction (SNCR).
In a second method, a catalyst is used to speed the reaction, which is known as Selective Catalytic Reduction (SCR). This method may involve the use of a catalyst bed and NH3 being exposed to temperatures around 371° C. Subsequent to the reduction of NOx, the gas stream is cooled to temperatures in the range of about 132° C. to 188° C. In another variation of this method, a hydrocarbon is utilized instead of NH3 to speed the reaction, as described in Ind. Eng. Chem. Res., 2001, 40, pp. 515-521. In this variation, the catalyst bed and hydrocarbon may be exposed to temperatures of about 200° C. to 400° C.
The prior techniques concerning NO reduction generally deal with the catalytic reduction of NO, which are subject to the usual disadvantages of employing catalysts. These disadvantages include the expense of the catalyst, the problems with loss of activity during operations, the expense and difficulty of contacting the combustion effluents (e.g., combustion products) with the catalyst, the disintegration of the catalyst, the emissions of disintegrated catalyst as a pollutant and similar problems.
As an example, U.S. Pat. No. 2,845,335 describes a regenerative pyrolytic process that involves conversion of a vapor phase via catalytic and non-catalytic techniques. The process uses endothermic and exothermic steps to produce products, such as olefins and acetylenes, from the gaseous starting materials.
U.S. Pat. No. 3,900,554, incorporated herein by reference, discloses a method for reducing the concentration of NO from combustion effluents. The method describes a step of contacting an effluent stream containing NO contamination with a sufficient amount of ammonia in the presence of a sufficient amount of O2 and at a sufficient temperature to selectively reduce the NO from the combustion effluent.
U.S. Patent App. Pub. No. 2010/0293926 discloses a method for reducing the concentration of NO from combustion effluents. The method comprises the step of contacting an effluent stream containing NO contamination with a sufficient amount of hydrocarbon in the presence of a sufficient amount of O2 and at a sufficient temperature to selectively reduce the NO from the combustion effluent in a gasoline fueled spark engine using engine out hydrocarbons.
U.S. Pat. No. 5,985,222 discloses gas-phase methods and systems for reducing NOx emissions and other contaminants in exhaust gases, and industrial processes using the same. The hydrocarbon(s) autoignite and autothermally heat an exhaust gas from an industrial process so that NH3, HNCO or a combination thereof are effective for selectively reducing NOx autocatalytically. Preferably, the reduction of NOx is initiated/driven by the autoignition of hydrocarbon(s) in the exhaust gas. Within the temperature range of about 482° C. to 871° C., the introduced hydrocarbon(s) autoignite spontaneously under fuel-lean conditions of about 2 to 18% O2 in the exhaust gas. Once ignited, the reactions proceed autocatalytically, heating the exhaust gas autothermally.
U.S. Pat. No. 6,551,565 discloses a process for removing nitrogen oxides from flue gases of a fluidized catalytic cracking unit wherein the flue gas is introduced into a tertiary cyclone with percentages of separated solids being discharged in both the overflow and underflow from the cyclone, treating the overflow in a catalytic reduction unit to form an outlet gas and treating the underflow and outlet gas in a wet scrubber.
U.S. Patent App. Pub. No. 2003/0184320 discloses a tubular probe which has spaced-apart bands or patches of the same material as the probe body attached to the probe body by an electrically insulating, high temperature material. A series of cooling tubes are provided within the probe body to direct cold air to the regions near each band. One or more probes are placed in a furnace or boiler above an ammonia injection zone.
While the prior art describes various NOx reduction techniques, a technique for NOx reduction that involves combustion products associated with high temperature pyrolysis is needed. In particular, a need exists for managing the NOx from gases resulting from processes utilizing combustion to generate heat, such as from pyrolysis reactors, in particular in regenerative pyrolysis reactors wherein unusually high temperature combustion occurs.